Two principal capacitive sensing and measurement technologies are currently employed in most touchpad and touchscreen devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC.™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded object is a finger. The human body is essentially a capacitor to a surface where the electrical field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual disturbances induced by the presence of a finger, for example, can be determined. To effect accurate multi-touch measurements in a touchpad, however, it may be required that several finger touches be measured simultaneously. In such a case, row and column techniques for self-capacitance measurement can lead to inconclusive results.
One way in which the number of electrodes can be reduced in a self-capacitance system is by interleaving the electrodes in a saw-tooth pattern. Such interleaving creates a larger region where a finger is sensed by a limited number of adjacent electrodes allowing better interpolation, and therefore fewer electrodes. Such patterns can be particularly effective in one dimensional sensors, such as those employed in IPOD click-wheels. See, for example, U.S. Pat. No. 6,879,930 to Sinclair et al. entitled Capacitance touch slider dated Apr. 12, 2005.
The second primary capacitive sensing and measurement technology employed in touchpad and touchscreen devices is that of mutual capacitance, where measurements are performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. Mutual capacitance technology is employed in touchpad devices manufactured by CIRQUE.™ In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In some mutual capacitance measurement systems, an array of sense electrodes is disposed on a first side of a substrate and an array of drive electrodes is disposed on a second side of the substrate that opposes the first side, a column or row of electrodes in the drive electrode array is driven to a particular voltage, the mutual capacitance to a single row (or column) of the sense electrode array is measured, and the capacitance at a single row-column intersection is determined. By scanning all the rows and columns a map of capacitance measurements may be created for all the nodes in the grid. When a user's finger or other electrically conductive object approaches a given grid point, some of the electric field lines emanating from or near the grid point are deflected, thereby decreasing the mutual capacitance of the two electrodes at the grid point. Because each measurement probes only a single grid intersection point, no measurement ambiguities arise with multiple touches as in the case of some self-capacitance systems. Moreover, it is possible to measure a grid of n×n intersections with only 2n pins on an IC.
Despite the advantages of a mutual capacitance measurement system, however, such a mutual capacitance grid arrangement is generally better suited to touchpad applications than touchscreen applications. In many touchscreen designs, for example, each of the rows and columns of electrodes requires its own layer of indium tin oxide (ITO). Using stacked layers of ITO can result in an excessive amount of light being absorbed by, or otherwise not transmitted through, a display, which decreases display brightness. In addition, with volume at such a premium in small handheld devices, anything that can be done to decrease the footprint, volume or thickness of a device is helpful.
Further complicating things is the fact that LCD displays are often employed in conjunction with overlying capacitive touchscreens. Because the individual LCD pixels used to form such displays are arranged in rows and columns, undesirable Moiré patterns or other types of optical interference patterns can be generated if the overlying touchscreen includes electrode elements that are also arranged in vertical columns and horizontal rows.
What is needed is a capacitive measurement system that may be employed in touchscreen and touchpad applications that is capable of distinguishing between multiple touches, does not absorb or otherwise excessively impede the transmission of light therethrough, has a small footprint, volume or thickness, and is highly sensitive to small touch objects being brought into proximity thereto.